1. Field of the Invention
This invention relates to an air-fuel ratio control system for an internal combustion engine, and, more particularly, to air-fuel ratio control system for an engine which is supplied with fuel evaporated in a fuel tank and collected.
2. Description of Related Art
Air-fuel ratio control systems of this kind have a canister positioned between an air intake passage and a fuel tank and a purge valve positioned between the canister and the air intake passage. This purge valve is opened according to openings of an engine throttle valve to supply evaporated fuel into the engine so as to burn the evaporated fuel keeping an air-fuel ratio less changed. Such an air-fuel ratio control system is known from, for instance, Japanese Unexamined Utility Model Publication No. 60-33316.
The air-fuel ratio control system described in the above-mentioned publication has the advantage that evaporated fuel is effectively used and, since supply of evaporated fuel does not cause a rapid change in air-fuel ratio of a fuel mixture, the evaporated fuel is burned without bringing the engine into undesirable operating conditions. Because of supplying evaporated fuel without rapidly changing the air-fuel ratio, this air-fuel ratio control system is referred to a linear purging system.
However, because configurations of the conventional linear purging systems of this type premise that purging is allowed only in a range in which air-fuel ratio feedback control is performed, in other words, that when a large quantity of evaporation, which disables a proper feedback control of air-fuel ratio, is caused, a certain quantity of the evaporated fuel is released off into air, the evaporated fuel is not always effectively used.
Accordingly, in addition to the elimination of draining of evaporated fuel which is required from a viewpoint of strict environmental regulations, it is required to consume almost all of evaporated fuel in the whole range of engine operating conditions. On the other hand, in order to trap the entire quantity of evaporated fuel from a fuel tank without releasing it partially, a large capacity of canister is essential.
However, as previously described above, because the linear type of purging systems premise air-fuel ratio feedback control, they are hard to expand the purging quantity of fuel not accompanying the deterioration of running performance of vehicles. For instance, a purging system, described in, for instance, Japanese Unexamined Patent Publication No. 2-130240, which lets a control value learned during idling under on-purging is reflected in non-idling according to the quantity of intake air. However, the quantity of fuel re-evaporated from a canister is not simply determined based on the quantity of intake air only, and consequently, it does not function with a high accuracy during non-idling.